Middle Vascular Layer

Question 1

Middle vascular layer

Answer 1

1. Choroid
2. Ciliary body
3. Iris

Question 2

Choroid

Answer 2

Structure: this is a thin, dark layer containing pigments and blood vessels
Functions: 1. The pigments absorbs excessive rays of light preventing their reflection within the eye which would cause blurring
2. The blood vessels supply food and oxygen to the cells of the retina

Question 3

Ciliary body

Answer 3

Structure: this thickened part is an extension of the choroid and contains circular muscles.
Functions: the ciliary muscles play an important role in controlling the curvature of the lens during accommodation

Question 4

Iris structure

Answer 4

Structure: this is a continuation of the choroid and it forms a circular coloured curtain with a hole, the pupil in the centre.

• this annular structure contains brown pigments
• two sets of involuntary muscles occur in the iris: circular and radial. They work antagonistically. When one contracts the other rests.
• the pupil is a circular opening in the middle of the iris.
• the colour of the iris depends on the amount of brown pigment present.

Question 5

Iris function

Answer 5

The iris controls the amount of light entering the eye by pupillary mechanism which is a reflex action.
In bright light:
- circular iris muscle contracts
- the radial iris muscle relax
- the size of the pupil reduces
Less light enters the eye preventing that ultra-violet rays from damaging the retina
In dim light:
- radial muscles contract
- the circular muscle relaxes
- the pupil dilates
This allows the maximum amount of light to enter the eye for the clearest vision